The United States pledged Thursday to deepen cooperation with allies South Korea and Japan on deterring the North Korean nuclear threat, working to ramp up pressure following worrying provocations. Leaders of the three countries urged the world community to vigilantly enforce new U.N. sanctions.

President Barack Obama didn't disclose what further steps the countries might take as he met with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye on the sidelines of a nuclear security summit. But he said the countries had directed their teams to work together to help bring about a denuclearized Korean Peninsula.

"We are united in our efforts to deter and defend against North Korean provocations," Obama said. "We recognize that our security is linked."

Park, whose country has been repeatedly threatened by Pyongyang, warned that North Korea would face even stiffer sanctions and more isolation if it engaged in any further provocative acts. Speaking through a translator, she said the mere fact the three leaders were huddling to discuss North Korea carried "huge significance."

Though nuclear terrorism and the Islamic State group top this year's agenda, concerns about North Korea's nuclear weapons program are also commanding focus as the two-day summit gets under way. Those long-simmering concerns have escalated of late following the North's recent nuclear test and rocket launch.

China's influence over North Korea will be front and center later in the day when Obama sits down with President Xi Jinping. The White House said that meeting was also an opportunity for Obama to press U.S. concerns about human rights and China's assertive territorial claims in waters far off its coast.

Though frictions with China remain high, the U.S. was encouraged by China's role in passing stringent new U.N. sanctions on North Korea, its traditional ally. Now the U.S. is pressing Beijing to implement those sanctions dutifully.

The U.S. and South Korea have been discussing whether to deploy a U.S. missile defense system called THAAD, or the Terminal High-Altitude Area Defense, in South Korea to counter the threat from the North. China has resisted that step out of concern it would also give the U.S. radar coverage over Chinese territory, and Russia opposes it as well.

In North Korea, meanwhile, the government has been churning out regular propaganda pieces condemning the U.S. and South Korea, while warning it could launch a pre-emptive strike against South Korea or even the U.S. mainland at any time.

For Obama, the summit's offers a last major chance to focus global attention on disparate nuclear security threats before the president's term ends early next year.

For years, pressing security crises in the Middle East have overshadowed Obama's goal of expanding U.S. influence and engagement in Asia, with the North Korean threat another unwanted distraction. Though the U.S. and China have struck sweeping agreements on climate change, they've remained at odds on many economic issues. Obama has also been unable to get Congress to ratify the Asia-Pacific free trade deal his administration painstakingly negotiated.

Obama also planned to meet Thursday with French President Francois Hollande, amid steep concerns about terrorism in Europe following Islamic State-linked attacks in Paris and Brussels. The summit continues on Friday with a special session focused on preventing IS and other extremists from obtaining nuclear materials and attacking urban areas.

Some of the 2,000 metric tons of highly enriched uranium and separated plutonium being used in civilian or military programs worldwide could be turned into a nuclear bomb if stolen or diverted, the White House warned. Fewer than half of the countries participating in the summit have even agreed to secure sources of radiological material that could be used for a dirty bomb, though more countries are expected to announce commitments during the summit to tighten controls.

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Associated Press writer Matthew Pennington contributed to this report.

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Reach Josh Lederman on Twitter at http://twitter.com/joshledermanAP